After you apply the cumulative update 8 in a Microsoft Exchange Server 2013 environment, you experience a large Input/Output Operations Per Second (IOPS) Write increase for FolderUpdateGroup on storage. This may cause delays in email delivery to the mailbox database.
This is a known issue for FolderUpdateGroup.
To work around this issue, disable indexing for the mailbox database by running the Set-MailboxDatabase "Mailbox Database" -IndexEnabled $false cmdlet.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.